Many concrete structural units are precast formations made at a location away from the construction site. This is done to provide uniform and high quality concrete formations which are difficult to form in the field. The precast formations are delivered to the job site and installed as part of the structure being formed. Such formed concrete units include, for example, beams, columns, exterior panels, pipes and culverts, and the like. The formation of such precast structural units conventionally requires using forms which contact a major portion of the unit's concrete surface as it is being cast and cured. In addition, job-site formation of concrete columns and the like are prepared similarly.
In recent years, more and more concrete structures are being left exposed as architectural surfaces. Thus, the appearance of these surfaces is becoming more significant, particularly for vertically formed surfaces where texture and absence of surface blemishes are parameters designed into the fabrication of the concrete.
Concrete surfaces are also being required to be smooth and blemish free for integrity of the concrete, for protection of the embedded reinforcing steel, for uninterrupted flow of fluid over the concrete surface, and for avoiding collecting foreign material on the surface. These criteria are important where concrete is used in highway bridges and culverts, sewer pipe applications, food processing plants, that surfaces that are exposed to freeze-thaw environments, and anywhere that imperfections in the concrete surface reduce the durability and functionality of the concrete article.
During the mixing of the components used to form hydraulic cement compositions, such as mortars (small aggregate such as sand, hydraulic cement such as Portland cement, water) and concrete (small aggregate, large aggregate such as gravel, cement and water), air is entrapped in the composition. In certain instances, known cement additives are incorporated into the composition to enhance one or more of its properties. However, some cement additives, such as naphthalene sulfonate-based superplasticizers, are known to also cause additional entrapment of air in the composition.
Although small amounts of air voids are useful in concrete structures and structural units, e.g., to enhance its freeze-thaw characteristics, they should be made up of small voids which are uniformly distributed within the structure. Thus, during the casting of an unset concrete composition into a mold, the composition and/or the mold are vibrated in attempts to cause the solids to settle and push out excess air and to cause uniform distribution of the remaining air in the cast unit.
However, it is well known that the major problem in producing concrete with controlled uniformly smooth surfaces is the propensity of the fluid cast concrete to develop large (1-3 inch), irregular voids at the interface with the form walls. Such voids are commonly called "bugholes" in the industry. These large voids detract from the appearance of the structural s unit, and tend to reduce the surface strength. Bugholes are typically pockets of entrapped air which did not migrate Otto of the concrete before the concrete stiffened. Their occurrence can be controlled to some extent by altering the proportions of the various aggregates used in the concrete mixture, using aggregates of a particular size and shape, and designing the mixture to have optimum amounts of cement, water, and pozzolans. Unfortunately these procedures have drawbacks; they do not always work; an optimum mix design might be prohibited by concrete design specifications, and using the optimum materials might be costly.
Placement and compaction procedures also have a strong impact on the cosmetics of the cast surfaces. However, to conduct these procedures in a fashion to reduce the appearance of surface voids typically will slow the production rate to the point where the operation is unprofitable.
In some circumstances, the forms can be designed to aid in dissipation of the surface voids. Titis includes using special surface coatings or materials and orienting the form in such a way as to minimize constrictions. These might reduce the voids, but not uniformly and usually not economically.
The final method resorted to by the concrete producer is to form and cure the structural units in the customary manner, then fill the surface voids on the surface of the structural unit. This process involves hand-troweling a grout composition into each void. The process is laborious, expensive, and alters the color and texture of the concrete surface.
The problem of surface appearance in concrete structural units is compounded by the customary use of release agents coating the form surface. The release agent promotes the easy removal of the form from the hardened concrete structural unit after casting. However, commonly used release agents often comprise either heavy greases or partially oxidized oils, and s badly stain the surface of the concrete structural unit, necessitating postasting treatment of the surface to remove the stains.
Accordingly, it is an object of titis invention to provide cement admixtures and compositions which may be used to make structural units having a smoother surface appearance, without having to resort to the laborious correctional or preventative treatments of the current art.
It is a further object of the invention to provide a means of reducing the formation of bugholes, and also to allow the use of inexpensive release agents while alleviating the problem of surface staining caused by their use.